The present invention relates to an acid test kit and, more particularly, to an acid test kit used in vapor compression refrigerators and the like in which an indicator paper is held in a transparent tube-like fixture to monitor the acid level in the system simply, quickly and inexpensively.
Vapor compression refrigerators, heat pumps, and air conditioners must always be concerned with the presence of acids in the refrigerant which can severely shorten the life of both the compressor and the refrigerant. These acids can be formed by chemical reactions with components and/or materials of construction, lubricating oils, and/or impurities. The instability of the refrigeration, and thus the formation of acids, is accelerated by elevated temperatures which result from improper operation, such as a failed condenser fan, or clogged air flow path.
Checking the refrigerant and/or oil acid is a common maintenance procedure because acidic refrigerant can be cleaned up before permanent damage to the hardware and refrigerant occur. Acidic refrigerant will also result in hermetic compressor motor burn-out because the acid will degrade the motor winding""s electrical insulation. Moreover, the presence of acid indicates the existence of other decomposition products, such as non-condensable gases, which result in elevated pressures and increased compressor pressure ratios leading to reduced efficiency and overloaded compressor operation.
To avoid the above-mentioned problems, refrigeration systems are tested for acid content. Typically, the oil would be tested for acid, because the highest concentration of acid is found in the oil of a non-operating system (shut down). It is, however, much easier to test the refrigerant for acid instead of testing the oil for acid, since the refrigerant is pressurized and existing service valves provide an easy way of sampling the refrigerant. Testing of the refrigerant vapor, rather than the refrigerant liquid, of the system is much more convenient because testing the liquid refrigerant results in a much greater refrigerant loss and the exiting liquid will cause frostbite if not properly handled.
Visual sensors or indicators for use in detecting the corrosive state of a fluid in a heat exchanger system are known as seen, for example, in U.S. Pat. No. 5,127,433. A permanently installed sensor has a sight glass or window through which corrosiveness is determined by viewing a flap or ball displaying a color indicating either the need to change the fluid or to add corrosion inhibitors. Alternatively, corrosiveness can be indicated by a ruptured or broken diaphragm located between the sight glass and the fluid. This form of sensor is limited to applications such as automobile cooling systems where the sensor is provided in the overflow conduit or in the hot fluid conduit upstream of the radiator.
Humidity and corrosion indicators for packaged goods in which thin cobaltous chloride film is used as the sensing element are described in U.S. Pat. No. 3,084,658. An elastomeric grommet sealed by a transparent disk is inserted into an opening in a package wall. A disk impregnated with the cobaltous chloride is secured beneath a window and can be replaced.
With respect to closed refrigeration systems, other types of indicator systems are known for testing the presence and concentration of contaminants in a refrigerant. For example, U.S. Pat. Nos. 4,923,806 and 5,071,768 show apparatuses for testing liquid or vapor contaminants in a closed system regardless of whether the apparatus is operating or not. A disposable testing tube made of transparent material is used at the end of a compressor discharge line or elsewhere in the system. One section of the tube is provided with water removal and moisture indicating chemicals, such as cobaltous chloride and another section is provided with acid indicating chemicals such as a solution of bromophenol blue, ethanol and glycerol. This construction is relatively complicated and requires a separate, specially configured flow restrictor in addition to a tube holder, and an expensive testing tube in which the multiple contaminant testing chemicals and filter screens are permanently located.
Likewise, U.S. Pat. No. 5,377,496 shows an acid contamination indicator for closed loop vapor compression refrigeration systems in which the indicator is permanently or removably installed in the bypass line around the system compressor where the refrigerant is always in the gaseous phase. A casing has a visual indicator bed of bromophenol blue as the acid indicating medium which is contacted by the refrigerant after flowing through a filter and a flow restrictor orifice. Porous retainer disks are held against the bed by springs. Moreover, the indicator, which changes color when exposed to acids or bases, are solid, and thus they must be exposed to the test stream in some fashion. Accordingly, this solid indicator must be mixed with an inert substance to provide some porosity, contact surface area and increased volume and then packaged in a clear tube. The vapor refrigerant is then passed through the porous mixture arranged in a bypass loop between the suction and discharge ends of a compressor or in the main refrigerant flow path between the compressor discharge and a heat exchanger to observe a color change. Again, we have recognized that this is an unduly complicated construction which requires a substantial outlay for installation.
Another type of contaminant detector is marketed by Refrigeration Technologies of Fullerton, California under the trademark xe2x80x9cCHECKMATExe2x80x9d. A specific volume of gas passes through a detection tube at a predetermined termination pressure. However, an expensive sealed Pyrex detection tube containing a color-changing chemical and whose ends are pierced when fully assembled can only be used once even when the test is negative, and thus this approach entails considerable expense regardless of its technical merits.
In a vapor-compression system, refrigerant flows from the condenser to the expansion valve, where it flashes into a two-phase mixture and then enters the evaporator. Superheated refrigerant vapor, with some entrained oil, leaves the evaporator and is compressed in the compressor, before being condensed in the condenser to complete the cycle. When in chemical equilibrium, the majority of the acid in the system is contained in the oil, but acid is also present in the liquid and vapor refrigerant. The presence of water in the system, which is a very real possibility, causes an even greater concentration of the acid in the liquid rather than in the vapor. To further complicate the problem, the relative liquid and vapor acid concentrations are a function of the system""s liquid and vapor volume and therefore are system dependent. These factors all render the measurement of acid level in the refrigerant""s equilibrium vapor phase an uncertain indication of acid level in the compressor oil.
Although the acid content in the refrigerant vapor can not be exactly correlated to the compressor oil acid content, it is, however, accurate enough to indicate the relative status of the oil in the system. It is clear that when acid is detected in the vapor, the acid level in the oil is significantly higher. Independent of the system, it can be generally stated that a refrigerant vapor acid level about 1-2 parts per million (ppm) in the refrigerant vapor clearly means the oil acid content is high, and the system should be cleaned up to reduce the acid level. Furthermore, a refrigerant vapor acid of 10 ppm, clearly indicates the compressor""s oil acid level is well beyond safe operating levels and the system will fail shortly if the refrigerant and acid is not changed or cleaned.
We have discovered that a pH paper can provide a simple fast and extremely inexpensive way to test for acidity in a refrigeration system. It allows for testing with the system on or off, and in other applications as well. In addition, it does not have to be installed in a line but can be temporarily connected with, for example, a Schrader-valve to permit venting of a small amount of gaseous refrigerant to the atmosphere. This is a surprising discovery because a pH paper is typically used to measure the concentration of hydronium ions in an aqueous solution. However, an aqueous solution is not present in a refrigerant system. Table 5-30 of Lanqe""s Handbook of Chemistry (13th Edition) lists several chemical compounds for colorimetric pH indicators as follows:
These compounds change color depending on the form they take (for example, yellow when acidic and blue when basic). A pH paper is a filter paper totally impregnated with one or more of these indicator compounds, generally an organic compound, that is a weak acid with a certain pKa (pKa is defined as the negative log of the dissociation equilibrium constant). The pKa value determines the range of the indicator.
Specifically for our invention, we currently contemplate use of a pH paper manufactured by Micro Essential Laboratory, Brooklyn, N.Y. with a pH range of 1-6. This pH paper is impregnated with meta-cresol purple (trade name) also known as meta-cresolsulfonephthalein (chemical name). The structure of this chemical is given below: 
This indicator, meta-cresol purple, is red in acidic form and yellow in the basic form.
Our invention does not measure pH because the systems and the like with which the acid test kit is intended to be used do not contain an aqueous solution. We utilize pH paper because of the surprising discovery that it has an indicator which will react with the inorganic acid vapor present in the refrigeration system. Furthermore, filter paper impregnated with indicator solution is a commercially available product, namely pH paper, thus lowering the cost even more.
Thus, the present invention takes advantage of the low cost and ready availability of pH paper. If the indicator is in an acidic environment, the indicator will react with the acid and produce a red color. If the acid concentration is not enough to turn the indicator completely red, however, an intermediate color, between red and yellow (that is some shade of orange) will be observed. Therefore, the intensity of the color is concentration dependent. On the pH paper, a certain amount of the indicator is impregnated, and as the acid reacts with the indicator, the indicator""s color changes. When most of the indicator has reacted, a red color will be observed. Therefore, the intensity of the color change depends on what percentage of the indicator has been transformed (reacted) to the acidic form.
We have also found that in a refrigeration system the refrigerant vapor acid test should be performed from the suction (vapor) service port. If both a compressor suction-side and compressor discharge-side vapor connection is available, we have further recognized that the lower-pressure suction side should be used and the system should be operating in order to minimize the amount of oil discharged with the refrigerant vapor and will also serve to provide a more acidic sample. When the vapor-compression compression system is operating, the liquid refrigerant with some dissolved oil is vaporized in the evaporator, resulting in an acid vapor refrigerant flow with entrained liquid oil droplets.
The acid level of the vapor stream during operation is is not at equilibrium but instead is essentially the same as the acid level of the liquid that the refrigerant was flashed or evaporated from. Because the vapor is safer and much easier to sample, the sampling of the vapor phase during system operation is a much easier, safer, faster and better approach, as long as the system is operating, and will provide essentially the same accuracy as sampling of the liquid. In other words, although the equilibrium concentration of the acid in the vapor is lower than the acid concentration in the liquid refrigerant when the system is off (and would, therefore, provide lower and inaccurate pH readings), when the system is running (i.e., compressor operating, refrigerant flowing), the liquid refrigerant with acid and dissolved oil is flashed (evaporated) into superheated refrigerant vapor and entrained oil, and the acid is carried along with this vapor so that the acid level of this vapor is essentially the same as the acid level in the liquid refrigerant. Nevertheless, the acid test kit can be used in a system which has been shut off or in relation to a tank of recovered refrigerant. For those non-operational systems which need to be repaired, the oil can be tested directly (or simply changed) while the system is being repaired and thus does require an acid indicator which is safe and easy to use.
It is, therefore, an object of the present invention to provide an accurate, yet simple and inexpensive, acid test device which can sample the refrigerant""s acid level, by sampling the refrigerant vapor, from existing system service valves, or from a refrigerant recovery tank, to provide an indication of the condition of the refrigerant and therefore the condition of the system.
In order to determine the pH of the vapor sample, it is necessary for a known amount of refrigerant be used in the test. We have further developed a simple method of determining this known amount of refrigerant. All systems typically have service valves with valve core depressors (often referred to as Schrader-valves). These valves, like automobile tire-valves, are opened when a valve core is depressed, usually by the device being attached to the valve. For refrigeration systems, these types of service valves with valve core depressors are used in several standard sizes, with xc2xcxe2x80x3 being the most common and xe2x85x9cxe2x80x3, xc2xdxe2x80x3, and ⅝xe2x80x3 also used. For a given valve size, these service valves have a known flow cross-section. In order to determine the pH of the vapor sample, it is necessary to use a known amount of refrigerant. The present invention also uses these service valves as the flow metering device. In addition, for a given refrigerant, the system pressures are known, from the saturation pressure temperature correlation for the refrigerant. Therefore the combination of the known cross-section orifice area (service valve cross-sectional flow area) and known pressure can be used to calibrate refrigerant flow and therefore to calibrate acid level with the time necessary to react the indicator, that is to obtain a specific color change on the indicator.
The present invention advantageously uses a readily available, inexpensive indicator paper held in a transparent tube-like fixture to monitor the acid level. Indicator chemistry reaction is essentially a function of acid level and exposure time in an essentially linear fashion. That is, half the acid level exposed for twice the time will result in the same indicator reaction. Therefore, as discussed above, the effect of refrigerant flow must be considered in determining acid level. The present invention uses a standard refrigeration service valve with valve-core depressor (Schrader-valve), an industry-standard service valve, which is already present in essentially all refrigeration systems, as the natural throttling or metering device on the system. The system pressure depends only on the system refrigerant, thus advantageously allowing performance tables to be developed for each refrigerant.